Retire the bit of config.fish that detected missing builtin string

It's been a few years.

share/config.fish

@@ -16,23 +16,6 @@ function __fish_default_command_not_found_handler
 end
 
 if status --is-interactive
-    # The user has seemingly explicitly launched an old fish with too-new scripts installed.
-    if not contains -- "string" (builtin -n)
-        set -g __is_launched_without_string 1
-        # XXX nostring - fix old fish binaries with no `string' builtin.
-        # When executed on fish 2.2.0, the `else' block after this would
-        # force on 24-bit mode due to changes to in test behavior.
-        # These "XXX nostring" hacks were added for 2.3.1
-        set_color --bold
-        echo "You appear to be trying to launch an old fish binary with newer scripts "
-        echo "installed into" (set_color --underline)"$__fish_data_dir"
-        set_color normal
-        echo -e "\nThis is an unsupported configuration.\n"
-        set_color yellow
-        echo "You may need to uninstall and reinstall fish!"
-        set_color normal
-        # Remove this code when we've made it safer to upgrade fish.
-    else
     # Enable truecolor/24-bit support for select terminals
     # Ignore Screen and emacs' ansi-term as they swallow the sequences, rendering the text white.
     if not set -q STY
@@ -48,7 +31,6 @@ if status --is-interactive
         set -q fish_term24bit
         or set -g fish_term24bit 1
     end
-    end
 else
     # Hook up the default as the principal command_not_found handler
     # in case we are not interactive